No you cannot. They are not* mutually exclusive--take the implications of each one.
a)If you believe there is no god, then you accept (know) there is no god.
b)If you accept (know) "god is unknown or unknowable" then you believe that you never will be able comprehend god or know he does or does not exist.
The wikipedia "atheistic agnosticism" or "agnostic theism" is a bullshit way of putting... A person who believes there is no god but does not know it or a person who knows there is a god but does not believe it. If you say there is a god, then you believe there is a god. It is illogical know something and not believe it or to not know and believe something.
Walk up to someone and say "I believe there is a god" then say "I don't know if he exists". They would LAUGH at you...
c)Argument Restated/Rephrased:
The agnostic pleads ignorance. "God is unknown or unknowable"
The atheist says "There is no god"
Either you know there is no god or plead ignorance... cannot be both. You know there is no god and "god is unknown or knowable"? Not logical.
Atheist take: Atheist says there is no god then god is NOT "unknown/unknowable" for God is known--he does not exist. Thereby dispelling the agnostic belief.
Agnostic take: Agnostic says "god is unknown/unknowable" then he knows not if God does or does not exist. To state god does not exist would be to say God is known. Thereby dispelling their agnostic belief.
Believing is not the same thing as knowing, even though the definition of knowledge is mmm... blurry (?) .-.
a) If I don't believe there is a God that does not mean I have true knowledge about being right, so in order to keep my intellectual honesty I can't rule out the possibility of being wrong since what I believe/disbelieve will not make things true.
b) If I say "God
is unknown" is because I don't find enough evidence or arguments to assert any claim about the existence of such thing however that would not mean it
will remain unknown nor stop me from taking a stance about it (a stance that may change in light of new evidence).
It is like the Russell's teapot: I don't believe there is a teapot revolving around the sun but just because of that I can't say I know there isn't one. If I say I do know just because of my belief, my knowledge would not necessarily be true and would not be justified.
c) There are different types of atheism. That is just strong atheism, there is a difference between saying "I don't believe in gods" and "There is no God" because in the latter, you are stating an affirmation in a logical sense (despite expressing it in a grammatically negative way), therefore it acquires the burden of proof. An agnostic atheist does not make such affirmations.
* I guess this was a typo...